1. Field of Invention
The invention relates to multiple specimen cryotransfer holders for electron microscopes and especially to side-entry holders for transmission electron microscopes.
2. Description of Prior Art
Traditional resin embedding methods are effective in preserving most of the fine microstructural detail of biological specimens when examined in the electron microscope. Unfortunately, however, cell chemistry is completely disrupted. For these and other reasons, there is growing interest in ultra-rapid freezing of specimens as a method for preparing biological specimens. While this approach has many advantages, there are technical problems to be overcome; not only must the specimen be frozen very rapidly to minimize damaging ice crystal growth but once frozen, the specimen must be kept cold (typically .about.-180.degree. C.) at all times.
Cryotransfer holders have been developed which keep the specimen cold and prevent frost forming on the specimen during the transfer of the holder from the specimen-loading workstation into the microscope. Typically, once the specimen has been loaded into the holder, a close-fitting cryo-shield, at a similar temperature to the specimen, is moved into position to completely cover the specimen during transfer. Once inside the microscope, the shield is retracted.
In a known cryotransfer holder manufactured by Gatan Inc., the inner wall of a dewar containing liquid nitrogen is connected to the tip of the specimen holder by a copper conductor. The copper conductor is mechanically supported by low conductivity bearings within a tube which connects the outer wall of the liquid nitrogen dewar to the holder tip. The space between the outer tube and the copper conductor is contiguous with the space between the inner and outer walls of the liquid nitrogen dewar and is evacuated to provide thermal insulation. The greater part of this surrounding tube is made from steel to provide mechanical strength while the part adjacent to the holder tip is made from plastic to provide thermal insulation. By this means, the whole tip of the holder is cooled, unlike other transmission electron microscope specimen cooling holders where only a small specimen cradle is cooled. Problems of insulation are thereby minimized since more than adequate thermal insulation is provided by the low thermal conductivity silica rod in the holder tip which is the mechanical bearing to the transmission electron microscope stage. A thin rod sliding through the center of the copper conductor is used to move a close fitting cryoshield over the specimen.
A major problem in the observation of specimens inside the electron microscope at very low temperature is the build up of water ice on the specimen. This can be minimized by anticontaminators. However, most water vapor enters the microscope through the airlock with the specimen. Therefore, once the specimen is inside the microscope, it is usual to leave the protecting cryo-shield in position for as long as one hour while the microscope pumping system and the microscope anticontaminator remove the residual water vapor.
Heretofore, cryotransfer holders have only been capable of transferring one specimen at a time. Clearly, if users wish to obtain the best results, the number of specimens that can be observed is severely limited by the lengthy waiting period required after loading the specimen into the microscope. Most users, therefore, would find a significant advantage in being able to simultaneously transfer several specimen into the microscope.